unify add benchmark format (#28891)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28891
as title
Test Plan:
```
buck run mode/opt //caffe2/benchmarks/operator_benchmark/pt:add_test
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: add
# Mode: Eager
# Name: add_M64_N64_K64_cpu
# Input: M: 64, N: 64, K: 64, device: cpu
Forward Execution Time (us) : 125.279
...
Reviewed By: hl475
Differential Revision: D18226789
fbshipit-source-id: 0cc51c6691533b02f662d4b6108916455f3a5b95
